1. Field of the Invention
The present invention relates to a flash apparatus to be used as an artificial light source in taking a photograph, and more particularly, to a flash apparatus capable of making high-speed repeating light emission.
2. Description of Related Art
Flash apparatuses are popularly used in taking photographs, as artificial light sources for illuminating objects to be photographed. Some of the flash apparatuses are arranged to permit selection of the so-called flat light emission mode, in which a light emitting action is repeated at a high speed.
Meanwhile, a self-maintaining switch element such as a thyristor is used as an electric element for controlling various circuit actions in the flash apparatuses. For example, the self-maintaining switch element is used for controlling the coil connecting state (switching of coils) of the discharge loop of a main capacitor, for controlling the acting timing of a trigger circuit (starting of trigger), etc.
FIG. 5 is an electrical circuit diagram showing an example of a flash apparatus arranged to permit selection of the flat light emission mode and also to use, as a coil switching element, a thyristor which is one of self-maintaining switch elements.
Referring to FIG. 5, the flash apparatus has a DC high-voltage power supply 101 composed of a DC low-voltage power supply, such as a battery, and a DC/DC converter circuit, and a main capacitor 102 which is connected to both ends of the DC high-voltage power supply 101. To both ends of the main capacitor 102 is connected a series circuit composed of coils 103 and 104 which are a plurality of current-limiting elements, a flash tube 105, and a control element 106, which is, for example, an insulated-gate bipolar transistor (hereinafter referred to as IGBT), arranged to control the light emitting action of the flash tube 105 performed by consuming the electric charge accumulated in the main capacitor 102.
Further, the flash apparatus includes also a self-maintaining switch element 107, such as a thyristor, which is connected to both ends of the coil 104 in the forward direction, transistors 108 and 109 which are arranged to control the on- and off-actions of the thyristor 107, resistors 110 and 111, a capacitor 112 which is parallel-connected to the resistor 111, a control circuit 113 which is arranged to control the on- and off-actions of the transistor 109, a trigger circuit 114 which is arranged to excite the flash tube 105, a light-emission control circuit 115 which is arranged to control the action of the IGBT 106, and a diode 116 which is connected, in the reverse direction, to a series circuit composed of the coils 103 and 104 and the flash tube 105.
The coils 103 and 104 and the thyristor 107 are provided for controlling the rise characteristic of a discharge current of the electric charge of the main capacitor 102 flowing through the flash tube 105 at the time of discharge, so as to control the rise characteristic of light emitted from the flash tube 105.
More specifically, either a first discharge path in which only the coil 103 is inserted or a second discharge path in which both the coils 103 and 104 are inserted and which differs in impedance value from the first discharge path is selected as a path for the discharge of the electric charge of the main capacitor 102 through the flash tube 105, in accordance with the on- and off-actions of the thyristor 107. By virtue of this selection of the discharge path, the light emitting mode of the flash tube 105 can be selectively controlled between a normal light emission mode in which the waveform of light emission has a steep rise characteristic and a continuous light emission mode in which the waveform of light emission has a gentle rise characteristic and the light emission is continuously repeated at a high speed, i.e., the flat light emission mode.
With the flash apparatus configured in the above-stated manner, in setting the normal light emission mode, the transistor 109 is first made to be turned on by a control signal outputted from the control circuit 113. This causes, through the transistor 108, the resistor 110, etc., a turn-on voltage to be supplied to a gate, serving as a control electrode, of the thyristor 107, so that the thyristor 107 is turned on.
In this state, when the flash tube 105 is excited by the action of the trigger circuit 114 and the IGBT 106 which is a control element is turned on by the light-emission control circuit 115, the electric charge accumulated in the main capacitor 102 is discharged to the flash tube 105 through the coil 103, the thyristor 107 and the IGBT 106. In other words, the first discharge path, in which the coil 104 is not inserted, is thus selected as the discharge path for the main capacitor 102. The flash tube 105 then emits light by consuming the electric charge of the main capacitor 102 discharged through the first discharge path. At a result, the waveform of light emission of the flash tube 105 comes to have a steep rise characteristic.
In setting the flat light emission mode, on the other hand, the transistor 109 is made to be turned off by stopping the supply of a signal from the control circuit 113, so that any turn-on voltage is prevented from being applied to the gate, which is a control electrode, of the thyristor 107, in such a way as to keep the thyristor 107 in an off-state.
Under this condition, when the flash tube 105 is excited and the IGBT 106 which is a control element is turned on, the electric charge of the main capacitor 102 is discharged to the flash tube 105 through the coils 103 and 104 and the IGBT 106 without passing through the thyristor 107, unlike in the case of the normal light emission mode described above. In other words, the second discharge path in which the coils 103 and 104 are inserted is selected as the discharge path for the main capacitor 102. The flash tube 105 then emits light by consuming the electric charge of the main capacitor 102 discharged through the second discharge path. As a result, the waveform of light emission of the flash tube 105 has a gentle rise characteristic.
However, during the setting of the flat light emission mode in which the electric charge of the main capacitor 102 is discharged through the second discharge path by keeping the thyristor 107 in an off-state, if the cycle of the flat light emission is such a cycle that the next period of light emission begins while a gas which is sealed in the flash tube 105 still remains in an ionized state in a terminating stage of the last period of light emission although light is no longer emitted, the thyristor 107 might erroneously be turned on by a voltage induced at the coils 103 and 104 when the IGBT 106 which is a control element is turned on in the second and subsequent periods of light emission.
In the case of the flat light emission in the above-stated cycle, the first period of light emission can be made in a normal manner, because no energy has been accumulated as yet at the coils 103 and 104 to cause any fluctuations in potential between the cathode and the anode and between the cathode and the gate of the thyristor 107 due to the turning-on action of the IGBT 106.
However, when the IGBT 106 is turned on for the second period of light emission, the turning-on action of the IGBT 106 causes an abrupt drop of the cathode potential of the thyristor 107 to a ground level, so that a counter electromotive force induced at the coil 103 when the IGBT 106 is turned off to terminate the first period of light emission and a counter electromotive force generated at the coil 104 due to the sudden supply of energy from the main capacitor 102 are applied between the cathode and the anode and between the cathode and the gate of the thyristor 107.
Accordingly, the potential between the cathode and the anode of the thyristor 107 and the potential between the cathode and the gate of the thyristor 107 rise. When the rise of potential between the cathode and the gate comes to exceed the turn-on voltage Vg of the thyristor 107, the thyristor 107 is erroneously turned on, despite the fact that the thyristor 107 is not normally controlled to be turned on based on the turning-on action of the transistor 109 by the control circuit 113.
If the cycle of the flat light emission is such a cycle that the next period of light emission begins while a gas which is sealed in the flash tube 105 still remains in an ionized state although light is no longer emitted, the thyristor 107 remains in the erroneously turned-on state since a current flows to the thyristor 107 through the flash tube 105 which is in the ionized state.
With the thyristor 107 erroneously turned on, the electric charge of the main capacitor 102 is discharged to the flash tube 105 through the first discharge path without passing through the coil 104. The waveform of light emission of the flash tube 105 then comes to have a steep rise characteristic. In other words, the waveform of light emission of the flash tube 105 fails to become the waveform having a gentle rise characteristic normally expected to be obtained by the discharge through the second discharge path including the coil 104 in the flat light emission mode. As a result, it becomes impossible to carry out a flat light emitting action in a stable manner. Besides, in such a case, the IGBT 106 which is a control element tends to be broken with the discharge current of the steep rise characteristic repeatedly caused to flow to the IGBT 106.
FIG. 6 is an electric circuit diagram showing an example of a flash apparatus which is arranged to permit selection of the flat light emission mode and which uses, as a trigger starting switch element, a thyristor which is one of self-maintaining switch elements. In FIG. 6, all component elements denoted by the same reference numerals as in FIG. 5 have the same functions as the corresponding component elements of the flash apparatus shown in FIG. 5. Further, in this example, the flash apparatus does not include the thyristor 107 for switching of coils shown in FIG. 5.
In the case of the flash apparatus shown in FIG. 6, a series circuit composed of the coil 103 which is a current limiting element, the flash tube 105 and the IGBT 106 is connected to both ends of the main capacitor 102. The flash apparatus is provided with a trigger capacitor 117, a trigger transformer 118, a trigger thyristor 119 which is a self-maintaining switch element, and a resistor 120. To the gate, which is a control electrode, of the trigger thyristor 119, is connected a trigger generating circuit 121 for supplying a turn-on voltage (trigger signal) to the trigger thyristor 119 through a capacitor 122 and a resistor 123. Further, a diode 124 is connected between the cathode of the flash tube 105 and the trigger capacitor 117 for the purpose of quickly charging the trigger capacitor 117.
In the flash apparatus shown in FIG. 6, when the IGBT 106 is turned on by the light-emission control circuit 115 and the trigger thyristor 119 is turned on by the turn-on voltage output of the trigger generating circuit 121, the electric charge of the trigger capacitor 117 is discharged through the thyristor 119, the IGBT 106 and the trigger transformer 118. Then, the flash tube 105 is excited (i.e., the trigger action is effected) by a high voltage induced by the above discharge on the secondary winding side of the trigger transformer 118. The flash tube 105 is thus caused to emit light by consuming the electric charge of the main capacitor 102.
The light emitting action of the flash tube 105 comes to a stop when the IGBT 106 is turned off by the light-emission control circuit 115 at a suitable point of light emitting process of the flash tube 105.
At this point of time, the flash tube 105 does not instantly return to its stable initial state in which the inside-sealed gas of the flash tube 105 is not in an ionized state. The flash tube 105 returns to the initial state through a state in which, although no light is emitted, the inside-sealed gas still remains in the ionized state and a certain amount of current can be allowed to flow.
While the flash tube 105 is still in the process of returning to the initial state, therefore, the trigger capacitor 117 is charged with a current flowing through the flash tube 105 in the ionized state, the diode 124 and the trigger capacitor 117. Since the charging action on the trigger capacitor 117 is accomplished very quickly as it is performed through a charging path which does not include the resistor 120, which is an element of a high impedance value.
With the flash apparatus configured in the above-stated manner, the flash tube 105 can be normally excited by the discharging of the trigger capacitor 117 even when the next light emission is to be made in a very short period of time after the current light emission. In other words, the flash apparatus configured in the above-stated manner is arranged to be capable of preventing the flash tube 105 from being not normally excited due to an insufficient charging of the trigger capacitor 117.
With the flash apparatus set in the flat light emission mode, on the other hand, if the light emission by the flash tube 105 is to be repeated for second and subsequent periods in such a repeating cycle that the light emission of the current period begins while the flash tube 105 is still in the process of finishing the light emission of the preceding period and the inside-sealed gas of the flash tube 105 is still in an ionized state although no light is emitted, the flash tube 105, at the commencement of light emission for the current period, is in an ionized state in the same manner as when a trigger action is performed by discharging the trigger capacitor 117. Therefore, without necessitating the trigger action, the light emission for the current period can be allowed to start by just turning on the IGBT 106 which is a control element.
Therefore, the supply of the above-stated turn-on voltage to the gate of the trigger thyristor 119 by the trigger generating circuit 121 is arranged to be made only for the first period of light emission and to be not made for the second and subsequent periods of light emission. Such an arrangement effectively prevents generation of noises due to the triggering action performed by discharging the trigger capacitor 117 and thus has been considered to be advantageous for the electric circuit of the flash apparatus.
However, even with the flash apparatus arranged in this manner, there still remains the possibility that the trigger thyristor 119 might be erroneously caused to turn on by the charging voltage of the trigger capacitor 117 which is quickly charged, when the IGBT 106 is turned on.
An operating state in which an erroneous on-state of the trigger thyristor 119 is caused to occur as mentioned in the above manner is described below with reference to FIGS. 7(a), 7(b) and 7(c).
FIG. 7(a) is a timing chart showing an operating state of the IGBT 106. FIG. 7(b) is a timing chart showing a state of potential obtained between the ground and an anode, which is an electrode on the high potential side, of the trigger thyristor 119, which is a self-maintaining switch element. FIG. 7(c) is a timing chart showing a state of potential of the trigger thyristor 119 obtained between a cathode which is an electrode on the low potential side and a gate which is a control electrode.
The IGBT 106 is turned on at a point of time T0 as shown in FIG. 7(a). After that, when a driving voltage which is equal to or higher than a turn-on voltage Vg of the trigger thyristor 119 is supplied to the gate of the trigger thyristor 119 from the trigger generating circuit 121 between points of time T1 and T2, as shown in FIG. 7(c), the trigger thyristor 119 is turned on at the point of time T1. Therefore, as shown in FIG. 7(b), the potential between the ground and the anode of the trigger thyristor 119 abruptly drops to the ground level following a trigger action on the flash tube 105 by the discharge of the trigger capacitor 117 made through the trigger thyristor 119 which is turned on. Meanwhile, the flash tube 105 is caused by the above-stated trigger action to emit light by consuming the electric charge of the main capacitor 102.
When the IGBT 106 is turned off by the light-emission control circuit 115 at a suitable point of time T3 as shown in FIG. 7(a) while the flash tube 105 is in process of light emission, the light emitting action of the flash tube 105 comes to a stop and, at the same time, the trigger capacitor 117 is quickly charged through the flash tube 105 which is in an ionized state, etc. Accordingly, the potential between the ground and the anode of the trigger thyristor 119 comes to rise according to the progress of the charging action, as shown in FIG. 7(b).
Then, to make light emission for the next period, when the IGBT 106 is again turned on at a time point T4 at which the flash tube 105 is still in an ionized state, the flash tube 105 again begins to emit light by consuming the electric charge of the main capacitor 102.
In this instance, since the potential level of the cathode of the trigger thyristor 119 abruptly drops to the ground level at the same time, the charging voltage of the trigger capacitor 117 comes to be applied between the anode and cathode of the trigger thyristor 119. As a result of this, an upward change of potential is caused by a floating capacity component of the trigger thyristor 119 to take place between the cathode and gate of the trigger thyristor 119 after a point of time T4, as shown in FIG. 7(c).
Therefore, if the trigger capacitor 117 at the point of time T4 has been charged up to a charging voltage equal to or higher than a voltage Vt which enables a voltage equal to or higher than the turn-on voltage Vg to be applied to the gate of the trigger thyristor 119 by the floating capacity component of the trigger thyristor 119, the upward change of potential taking place after the point of time T4 between the cathode and gate of the trigger thyristor 119 reaches at least the turn-on voltage Vg at a point of time T5, as shown in FIG. 7(c). As a result of this, the trigger thyristor 119 is erroneously turned on, despite the fact that the normal turn-on control by supplying a turn-on voltage from the trigger generating circuit 121 is not performed at the point of time T5.
When the trigger thyristor 119 is turned on at the point of time T5, although the turning-on is an erroneous action, the potential between the ground and anode of the trigger thyristor 119 and the potential between the cathode and gate of the trigger thyristor 119 drop, after the point of time T5, in a characteristic manner shown in FIGS. 7(b) and 7(c). At the same time, a trigger action is performed by discharging, through the trigger transformer 118, the electric charge of the trigger capacitor 117 which has been quickly charged. This trigger action generates a noise.
This trigger action is unnecessary in view of the light emitting cycle, as mentioned above. However, after the trigger capacitor 117 is quickly charged up to the above-stated voltage Vt or above, the trigger action takes place every time the IGBT 106 is turned on for light emission of the next period. Under such a condition, a noise resulting from the trigger action tends to cause the light-emission control circuit 115 to malfunction, thereby making it impossible to adequately carry out a flat light emitting action in a stable manner.
Further, if the action of turning on the IGBT 106 for light emission of the next period is performed at a point of time when the charging voltage obtained by the quick charging action on the trigger capacitor 117 is lower than the above-stated voltage Vt, the trigger thyristor 119 is not immediately caused to be erroneously turned on by the turning-on of the IGBT 106. However, even in that case, since the quick charging action on the trigger capacitor 117 is carried on while the IGBT 106 is in an off-state, the charging voltage gradually rises. Therefore, when the IGBT 106 is turned on after the charging voltage reaches or exceeds the voltage Vt, the turning-on action of the IGBT 106 causes the trigger thyristor 119 to be erroneously turned on. Therefore, a noise would result also from the erroneous action of the trigger thyristor 119 like in the case of the first example of operation described in the foregoing, although the erroneous actions of the examples described above differ from each other in turning-on timing.